1. Field of the Invention
The present invention relates to an inverter transformer, and more particularly to an inverter transformer adapted to gain a high voltage by means of leakage inductance.
2. Description of the Related Art
In recent years, a liquid crystal display (hereinafter referred to as “LCD”) has been widely used as a display device for a personal computer or the like, replacing a cathode ray tube, what we call “CRT”. Unlike the CRT, the LCD does not emit light by itself, and therefore requires a lighting apparatus for lighting a screen, such as backlight or frontlight system. Cold-cathode fluorescent lamps (hereinafter referred to as “CCFL”) are generally used as light sources for the system and simultaneously discharged and lighted.
For lighting and discharging the CCFLs, an inverter circuit is generally employed, which generates a high-frequency voltage of about 60 kHz and about 1600 V at the start of discharging. The inverter circuit, after the discharge of CCFLs, steps down its secondary side voltage to about 600 V, which is necessary to keep CCFLs discharging. Up to now, the inverter transformer for use in the inverter circuit has been available in two types; that is, an open magnetic circuit structure using an I-core as a magnetic core, and a closed magnetic circuit structure.
With the open magnetic circuit structure, since the number of the inverter transformer increases with an increase of the number of the CCFLs by one-to-one ratio, the inverter transformer is increased in size as a whole, and the cost is pushed up. And, with the closed magnetic circuit structure, although a plurality of CCFLs can be discharged by one inverter transformer, variation in the discharging operation occurs between the CCFLs, and also the inverter transformer is damaged by excess current. The problem of the variation in the discharging operation between the CCFLs can be solved by inserting a ballast capacitor in series between the CCFLs, but this decreases power efficiency and increases variation in the CCFL current. Furthermore, this results in an increased number of components and increased cost of production.
A conventional inverter transformer intended to solve these problems is disclosed in, for example, Japanese Patent Application Laid-Open No. 2002-353044. FIG. 8 shows such an inverter transformer 20, which comprises a magnetic core 21 consisting of a substantially rectangular frame-core 22 (hereinafter referred to as “frame-core”) and two I-shaped inner cores 23a, 23b (hereinafter referred to as I-core). The inverter transformer 20 further comprises a primary winding 24, two secondary windings 25a, 25b, and two bobbins 26a, 26b which are of tubular structure with a rectangular cross section, and which have therearound the aforementioned two secondary windings 25a, 25b, respectively, and the aforementioned primary winding 24 provided corresponding to the two secondary windings 25a, 25b in common. Magnetic flux, which is generated by causing current to flow through the primary winding 24, flows through the I-cores 23a, 23b in the same direction thus forming two separate magnetic fluxes flowing respectively into two opposing sides 22a, 22b (magnetic paths) of the frame-core 22 without interfering each other, thereby enabling two CCFLs to be driven at the same time.
Thus, the inverter transformer, while having only one primary winding, has a plurality (two in the figure) of independent secondary windings sharing the one primary winding, and therefore two CCFLs can be lighted at the same time without installing two inverter transformers or two ballast capacitors as have been required conventionally. However, the following problem is associated with the inverter transformer. That is, in recent years the LCD of side edge type uses as many as six lamps, with three CCFLs disposed at its upper side and another three CCFLs disposed at its lower side. In this case, three of the inverter transformers discussed above are required in order to light the six CCFLs. This invites a cost increase, and also prevents downsizing of the apparatus.